The present invention relates to oscillation control apparatuses for ultrasonic oscillators for doing ultrasonic machining and, more particularly, to an oscillation control apparatus for controlling a drive frequency of an ultrasonic oscillator so as to render, zero, a phase difference between driving voltage and driving current of the ultrasonic oscillator.
Generally, as shown, for example, in FIG. 3 of the attached drawings, a ultrasonic oscillation device 1 comprises an oscillation circuit 2, which generates a pulse signal with a high free-running frequency f.sub.0. According to the pulse signal, a high-frequency pulse signal of the frequency f.sub.0 is outputted from a drive circuit 3 and is applied to an ultrasonic oscillator 5 via a transformer 4, so that the ultrasonic oscillator 5 is ultrasonic-oscillated at the drive frequency f.sub.0, thereby doing ultrasonic machining.
During ultrasonic machining, when a load is applied to the ultrasonic oscillator 5 for the machining, an oscillation frequency of the ultrasonic oscillator 5 is altered or changed from f.sub.0 to f.sub.1, whereby, as shown in FIG. 4, a phase shift occurs between driving voltage V and driving current I of the ultrasonic oscillator 5. Thus, an efficiency is reduced. For this reason, the oscillation frequency f.sub.1 of the ultrasonic oscillator 5 with respect to the free-running frequency f.sub.0 is detected by a piezoelectric element 6 or the like and is fed-back to the oscillation circuit 2, whereby the oscillation circuit 2 alters the free-running frequency from f.sub.0 to f.sub.1.
In the vibration control device 1 constructed as above, however, if there is a large difference between the free-running frequency f.sub.0 and the oscillation frequency f.sub.1 of the ultrasonic oscillator 5, the oscillation frequency is f.sub.1, but the phase difference between the voltage V and the current I is large so that optimum tracking is not done.